Thermal printing paper typically comprises a support layer coated with a thermally sensitive layer that darkens upon exposure to heat. The heat is applied to the thermal paper by passing the paper under a heated print element mounted in a print head. As the thermal paper is transported past the print head, the thermally sensitive layer is exposed by selective energization and heating of the print element so as to "print" on the thermal paper.
In one type of prior art thermal recording paper, the thermally sensitive layer is the outermost layer of the thermal paper and is in contact with the heated print element during printing. Several problems result from this arrangement. The thermally sensitive layer and the image therein are subject to physical and chemical degradation. Furthermore, the thermally sensitive layer can soften and stick to the print element during printing, thereby interfering with the desired paper movement and generating a significant amount of noise. Contact between the print element and the thermally sensitive layer can also cause portions of the thermally sensitive layer to peel off and leave a residue on the print element. When the thermal paper comprises labels upon which bar code patterns are thermally printed, an additional problem is that repeated scanning of the label by a bar code reading device may eventually degrade the label to a point where the bar code is unreadable or produces an incorrect reading.
In an effort to alleviate some of these problems, certain prior thermal paper and labels have included a protective layer comprising a water soluble polymer coated onto the thermal layer. Examples of prior protective layer ingredients include polyvinyl alcohol and various cellulose derivatives. The use of a protective layer comprising a water soluble polymer does provide some degree of physical and chemical protection for the thermally sensitive layer. However the water soluble polymer also significantly increases the sticking between the thermal paper and the print element during the thermal printing process. Sticking is a particular problem for thermal labels and other thermal papers on which bar code patterns are printed. In a bar code printing process, lines having a length up to two inches or more are printed in a direction transverse to the direction of paper movement past the print element. Sticking between the print element and the thermal paper can therefore result in large net forces that can lead to the sticking problems discussed above or to jamming of the printing apparatus.